1. Field of the Invention
This invention relates to treatments for cleaning boreholes in a subterranean formation and particularly to removing cuttings beds or build-up of fines and smaller sized wellbore cuttings from a deviated borehole during a drilling operation.
2. Description of Relevant Art
Rotary drilling methods employing drilling apparatus having a drill bit and drill stem have long been used to drill wellbores in subterranean formations. Drilling fluids or muds are commonly circulated in the well during such drilling to serve a number of functions, including cooling and lubricating the drilling apparatus, counterbalancing the subterranean formation pressure encountered, and removing drill cuttings from the formation out of the wellbore. In removing drill cuttings from the well, drilling fluids suspend the cuttings and carry them to the surface for removal from the well.
Drilling deviated and horizontal wells has become increasingly common in the oil and gas industry. In drilling such wells, gravity causes deposits of drill cuttings, and especially fines or smaller sized cuttings, to build up along the lower or bottom side of the wellbore. Such deposits are commonly called "cuttings beds." As used herein, the term "deviated" with respect to wells shall be understood to include any well at sufficient angle or deviation off of vertical that cuttings beds tend to form during the drilling operation. "Deviated" wells shall be understood to include without limitation "angled," "high-angled," "oval," "eccentric," "directional" and "horizontal" wells, as those terms are commonly used in the oil and gas industry. The terms "well," "wellbore" and "borehole" are synonymous unless indicated otherwise.
Cleaning (i.e., removing drilling cuttings from) a deviated well, particularly drilled at a high angle, can be difficult. Limited pump rate, eccentricity of the drill pipe, sharp build rates, high bottom hole temperatures and oval shaped wellbores can all contribute to inadequate hole cleaning. In turn, inadequate hole cleaning can lead to cuttings beds build-up in the wellbore, as commonly used drilling fluids sometimes fail to remove cuttings from such cuttings beds while circulating through the wellbore.
Buildup of cuttings beds can lead to undesirable friction and possibly to sticking of the drill string and is especially a problem in Expanded Reach Drilling and in wells using invert emulsion type drilling fluids.
Well treatments or circulation of fluids specially formulated to remove these cuttings beds are periodically necessary to prevent buildup to the degree that the cuttings or fines interfere with the drilling apparatus or otherwise with the drilling operation. Two commonly used types of fluids that have been applied with limited success are highly viscous fluids, having greater viscosity than the drilling fluids being used in the drilling operation, and lower viscosity fluids, having less viscosity than the drilling fluids being used in the drilling operation. Commonly, the drilling operation must be stopped while such treatment fluids are swept through the wellbore to remove the fines.
Alternatively, or additionally, special viscosifier drilling fluid additives have been proposed to enhance the ability of the drilling fluid to transport cuttings but such additives at best merely delay the buildup of cuttings beds and can be problematic if they change the density of the drilling fluid.
A mechanical operation for removing cutting beds has also been used wherein the drill string is pulled back along the well, pulling the bit through the horizontal or deviated section of the well. Dragging the bit stirs up cuttings in the cuttings bed to better enable the drilling fluid to transport the cuttings up the well. However, such dragging the bit can damage its gauge side and dragging the bit while rotating further reams the hole. Also, such "wiper trips" are time consuming which increases drilling costs for the well and delays the ultimate completion of the well.
An alternative mechanical operation for removing cuttings beds has been proposed that employs drilling with coiled tubing and injecting fluid into the wellbore through the tubing at a flow rate exceeding the flow rate range used for drilling, as discussed in U.S. Pat. No. 5,984,011, issued Nov. 16, 1999, to John G. Misselbrook and Graham B. Wilde. However, this operation calls for special equipment and requires that drilling be stopped during the treatment, resulting in delays and increased drilling costs.
There continues to be a need for improved methods and materials for removing deposits of fines and smaller cuttings or cuttings beds from the lower side of a deviated borehole.